When using a steam cycle to generate electricity, there needs to be a way to reject heat in order to condense the steam after it has gone through the turbine. Traditionally, this is done in a wet cooling tower where the heat is rejected by evaporating water. Wet cooling towers are very effective because water is an efficient heat transfer medium and the latent heat of vaporization of water is very high (˜970 Btu/lb). However, this still requires an enormous amount of water. In DOE's case 13 (an NGCC plant running at a constant 59° F.), the water consumption is listed as 1,831 gallons/minutes or almost a billion gallons per year (most of which is used in the wet cooling tower). In water stressed regions, this amount of water consumption may not be feasible. The alternative to wet cooling is dry cooling, often in the form of an air cooled condenser (ACC). In an ACC, air is used to condense the steam instead, and there is no net water usage.
The trouble with a dry cooling system is that its performance is worst (power production drops the most) in the summer during the times of highest consumer power demand when replacement power is most expensive. Dry cooling can be augmented by using some amount of water. For instance, water can be sprayed into an ACC, reducing the temperature of the air entering the ACC and increasing its performance (Maulbetsch 2007). Alternatively, a hybrid wet-dry cooling tower can be used. Here, the dry cooling tower handles most of the cooling load, throughout most of the year. However, when dry bulb temperature is hottest, and the dry tower is least effective, then the wet part takes over and handles most of the cooling load. However, both of these methods still consume some water, and in many places, due to a combination of permitting and water scarcity, a power plant may not be able to draw from any local water sources. A solution, is to recover the water of combustion, this water can then be used for augmented dry cooling while still maintaining zero net water consumption. Past studies have looked at recovering the water of combustion for cooling and they have used the water when it was generated, without storage.
Calcium chloride desiccant cycles have been proposed to recover water of combustion; for example the WETEX cycle (Folkedahl 2006 and 2009) that was developed for use with pulverized coal plants. Because it was designed for use with coal plants where there is no “free” hot, dry gas stream to drive the regeneration, the WETEX cycle used a combination of vacuum and regenerative heat exchange (recovery of the heat of adsorption) to drive the desiccant cycle. To withstand the vacuum forces, the entire system had to be made of metal, and to prevent corrosion metal components have to be made from stainless steel because the calcium chloride solution is so corrosive. As a result, the capital cost of the WETEX process was high.
Tokarz et al. (1978) reviewed previous dry and wet cooling tower systems. This study found that ammonia heat transport systems with either all wet or the deluge scheme for wet/dry cooling were potentially more cost-effective than the state-of-the-art power plant heat rejection systems. Levy et al. (2001) evaluated condensing heat exchanger technology for recovering moisture from flue gas from coal-fired power plants. The condensed water was used to replace some of the water that would otherwise be purchased for use in the wet cooling tower, and they found an economic benefit. Sijbesma et al. (2008) describe a membrane to separate steam from the flue gas. The steam (under vacuum) is then condensed by a dry cooling tower.
Copen et al. (2005) discussed a new technology that recovers the water vapor contained in power plant flue gas streams. Copen et al. (2005) analyzed a Liquid Desiccant Dehumidification System (LDDS or WETEX). A small storage unit allowed collection at night. Regeneration occurred under vacuum in a 316 SS flash drum, but the desiccant piping to the absorber, and to and from the flash drum (nominally at 1 psia and 100° F.) were made of SCH 80 chlorinated polyvinyl chloride (CPVC). Folkedahl (2006 and 2009) calculated the incremental cost of the Copen WETEX process with an air cooled condenser (ACC) to be $25.3 million greater than that of a wet cooling system. The water collected was less expensive than demineralized water ($0.05/gal). The process was studied for both coal fired power plants and for natural gas combined cycle (NGCC). For a 75 gpm WETEX system, the capital cost with a 500 MW NGCC was $8,771,000 and the cost per gal was 0.04$/gal, with water collection running 12 hours per day and 300 days per year.
The above prior art relating to power plants and generating electricity each suffer from at least one of the following limitations: the need to improve the overall efficiency of electric power generation, limited water in arid locations, the inability to get permitted to build new plants in arid climates that would require additional water for cooling due to water scarcity, high capital costs for vacuum-rated and corrosion-resistant parts used in exiting water collection processes.